Field of the Invention
The present invention relates to a method for controlling the heating of a catalytic converter being connected downstream of an internal combustion engine of a motor vehicle and having at least a short portion which can be electrically heated fast.
Catalytic converters are used to reduce the pollutant emissions from motor vehicles, and legal requirements in terms of the maximum upper limits of pollutants are becoming ever stricter. Since catalytic converters cannot reduce the emission of pollutants until after a certain warmup time has elapsed, without supplementary heating, catalytic converters for meeting especially stringent requirements are provided that can be heated electrically in at least some portions, so that they can be more quickly brought to a temperature required for the catalytic conversion. Such heatable honeycomb bodies have meanwhile been developed to such an advanced extent that models with low mass and a short length are available in which high rates of temperature change can be attained by electric heating. Given a suitably small surface area, that can even be carried out in the relatively cool exhaust gas stream in the cold-starting phase of an internal combustion engine. However, directly after starting of such an engine, the rate of temperature change in the exhaust gas system is nevertheless quite high. It is only after some time that the rate of change drops markedly. Other phenomena, such as the evaporation of water present beforehand in the catalytic converter and in the exhaust system, also affect the temperature behavior of the exhaust system during the cold-starting phase.
Various procedures for controlling such heatable catalytic converters are described, for instance, in U.S. Pat. No. 5,146,743. However, they do not take the above considerations into account, and they apply primarily to electrically heatable honeycomb bodies with low rates of temperature change. In fact, however, practical aspects which must also be taken into consideration limit the possibilities for controlling an electrically heatable catalytic converter. First, given the capacity of the vehicle battery, only a limited amount of electrical energy is available and it must be employed as economically as possible, since energy reserves must be kept adequate for various modes of operation of a motor vehicle, especially including driving short distances in the winter. For reasons of convenience as well as safety in dangerous situations, it is also not desirable, before the engine is started, to have to first wait through a heating period before starting can be performed. Such considerations have led to various heating strategies in the prior art, which begin directly after starting of the engine and introduce a high electrical output for 8 to 20 seconds. The purpose thereof is to put the catalytic conversion into operation as fast as possible, at least in a portion of the catalytic converter, in order to be able to utilize the chemical energy present in the engine exhaust gases for further heating of the catalytic converter as soon as possible.